Silicon-on-insulator (SOI) substrates have become desirable for many technologies, including metal-oxide semiconductor (MOS), complementary metal-oxide semiconductor (CMOS) devices, and advanced MOS junction-type field-effect transistors (MOSFETs). This is primarily because SOI fabrication processes result in increased packing densities, improved performances, better device isolations and reduced extrinsic parasitic elements, particularly those of the source and drain as well as leakage currents and thus significantly speeding up circuit operations.
As the name implies, SOI substrates generally include a thin layer of silicon on top of an insulator, wherein circuit components are formed in and on the thin layer of silicon. The insulator can be silicon oxide (SiO2), sapphire, or any appropriate material. For example, a sapphire substrate may be used as an insulator for target radio-frequency (RF) applications. In contrast, a bulk silicon wafer with an oxide layer as an insulator in the substrate may be used for target digital logic applications. In both cases, the insulator may serve to reduce junction capacitance between the heavily-doped devices and the lightly-doped bulk substrate, which may translate to less power consumption and greater circuit speed.
There are several techniques available for the fabrication of SOI substrates. One technique for fabricating SOI substrates is known as “separation by implantation of oxygen” (SIMOX), where oxygen is implanted below the silicon surface and the substrate is annealed to provide a buried silicon oxide layer with a silicon overlayer. The implantation time can be intensive and cost prohibitive. Moreover, the SOI substrate may be exposed to high surface damage and contamination. Another technique is known as “bond-and-etch-back” SOI (BESOI), where an oxidized wafer is first diffusion-bonded to an unoxidized wafer, and the backside of the oxidized wafer is then grinded, polished, and etched to the desired device layer. The BESOI approach may be free from the implant damage inherent in the SIMOX approach. However, a time consuming sequence of grinding, polishing, and etching may be required. Another technique is known as the hydrogen implantation and separation approach in which hydrogen is implanted into silicon with a thermally grown oxide to form embrittlement of the silicon substrate underneath the oxide layer. The implanted wafer may then be bonded with another silicon wafer having an oxide overlayer. The bonded wafer may be “cut” across the wafer at the peak location of the hydrogen implant by appropriate annealing.